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Vol. 283, Issue 2, 666-674, 1997
Departments of Neurosciences and Psychiatry, University of California at San Diego, La Jolla, California
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Abstract |
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 Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Prepulse inhibition (PPI) is a form of plasticity of the startle
response in which presentation of a weak stimulus immediately before an
intense startling stimulus reduces the resultant startle response.
Deficits in PPI, an operational measure of sensorimotor gating, are
observed in schizophrenia patients and can be modeled in rats by the
psychotogen phencyclidine (PCP). PCP-induced deficits in PPI in rats
are resistant to dopamine and serotonin antagonists but can be
antagonized by antipsychotics such as clozapine, olanzapine and
Seroquel. These latter antipsychotics have antagonistic actions at
several receptors, including alpha-1 and alpha-2
adrenergic, M1 muscarinic and 
-aminobutyric acid (GABA)-A receptors.
Although the direct actions of PCP are thought to be mediated by
noncompetitive antagonism of N-methyl-D-aspartate sites, PCP thereby
indirectly activates multiple neurotransmitter systems, including those
affected by the aforementioned antipsychotics. The present studies
examined the possibility that an antagonist action at a particular
receptor subtype might be responsible for the interaction between PCP
and the clozapine-like antipsychotics by testing whether a selective antagonist at alpha-1, alpha-2, M1 or GABA-A
receptors would prevent the PCP-induced deficit in PPI in rats. Animals
were pretreated with either the alpha-1 antagonist prazosin
(0, 0.5, 1.0 or 2.5 mg/kg), the alpha-2 antagonist RX821002
(0, 0.2 or 0.4 mg/kg), the M1 muscarinic antagonist pirenzepine (0, 10 or 30 mg/kg) or the GABA-A antagonist pitrazepin (0, 1.0 or 3.0 mg/kg)
and then treated with either saline or PCP (1.5 mg/kg). Because
prazosin was effective in blocking the effects of PCP, an additional
experiment tested the possibility that prazosin (0, 1.0 or 2.5 mg/kg)
would block the PPI deficits produced by the dopamine agonist
apomorphine (0 or 0.5 mg/kg). After drug administration, animals were
tested in startle chambers. PCP was found repeatedly to decrease PPI. Prazosin (1.0 and 2.5 mg/kg) blocked this deficit in two separate experiments but did not increase base-line PPI levels. The effects on
PPI were dissociable from changes in startle reactivity. Furthermore, prazosin did not antagonize apomorphine-induced disruptions of PPI,
which suggests that the antagonism of the PCP effect was not simply due
to a generalized improvement of deficient PPI. The antagonists for
alpha-2, for M1 and for GABA-A receptors had no effect on
base-line PPI or on PCP-induced disruptions in PPI. These findings
indicate that the PPI-disruptive effect of PCP may be mediated in part
by alpha-1 adrenergic receptors and that antagonism of
alpha-1 receptors may play a major role in mediating the
blockade of PCP-induced deficits in PPI by certain antipsychotics.
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Introduction |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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PCP
is a dissociative anesthetic, the clinical use of which was
discontinued after burgeoning reports of its psychotomimetic profile
(see Balster, 1987
). It has since been demonstrated that PCP is a
powerful psychotogen that in healthy humans elicits a wide range of
symptoms that resemble both the positive and the negative
symptomatology observed in schizophrenia, and acutely exacerbates
existing behavioral and cognitive impairments in schizophrenia patients
(Javitt, 1987; Allen and Young, 1978; Luisada, 1978; Snyder, 1988).
Ketamine, another dissociative anesthetic that is closely related to
PCP, has also been shown to produce psychotic symptoms and cognitive
disintegration in healthy volunteers (Krystal et al., 1994).
The deficits in central inhibitory mechanisms that likelycontribute to
psychotic symptoms in schizophrenia have been quantified using
manipulations of the startle response. The startle response is an
involuntary response to sudden intense stimuli that can be inhibited or
gated by presentation of a weak prepulse immediately before the
startling stimulus (Hoffman and Ison, 1980; Ison and Hoffman, 1983;
Braff and Geyer, 1990). This ubiquitous phenomenon of PPI is one form
of plasticity of the startle response and provides an operational
measure of sensorimotor gating, a central inhibitory or filtering
mechanism that is deficient in schizophrenia and schizotypal patients
(Braff et al., 1978; 1992; Bolino et al., 1994;
Cadenhead et al., 1993; Grillon et al., 1992). In
results consistent with its psychotomimetic profile, PCP has been found in rats to disrupt PPI (Geyer et al., 1990; Mansbach and
Geyer, 1989), mimicking the PPI deficits that are observed in the
aforementioned psychiatric populations. Moreover, ketamine disrupts PPI
in both rats and humans at subanesthetic doses, which is further
evidence of the ability of psychotomimetic compounds of this class to
impair sensorimotor gating (Karper et al. 1994; Krystal
et al., 1994; Mansbach and Geyer, 1991).
Although the impairment in sensorimotor gating produced by PCP is well
documented (Bakshi et al., 1994; Bakshi and Geyer, 1995;
Swerdlow et al., 1996; Johansson et al., 1994b;
Mansbach and Geyer, 1989; Wiley, 1994), the neuropharmacological
mechanisms by which this effect is exerted remain unclear. PCP is an
open channel blocker of the NMDA ionophore complex, acting as a
noncompetitive antagonist of the NMDA receptor. As a consequence, PCP
has myriad indirect facilitatory effects on monoaminergic and
cholinergic pathways in the brain. For example, PCP increases DA
release, augments ACh efflux, decreases acetylcholinesterase activity
and inhibits 5-HT and NE reuptake (Bowyer et al., 1984;
Hondo et al., 1994; Hutson and Hogg, 1996; Hori et
al., 1996; Johnson and Hillman, 1982; Rogers and Lemaire, 1991;
Smith et al., 1977). PCP has also been reported to increase
extracellular levels of GABA (Lillrank et al., 1994).
Antipsychotics such as clozapine have been shown to partially block
deficits in PPI produced by PCP and other noncompetitive NMDA
antagonists (Bakshi et al., 1994; Bakshi and Geyer, 1995; Swerdlow et al., 1996), in contrast to dopamine antagonists,
which do not block PPI disruptions induced by noncompetitive NMDA
antagonists (Geyer et al., 1990; Keith et al.,
1991; Hoffman et al., 1993; Swerdlow et al.,
1996). Because compounds such as clozapine have not been shown to
possess appreciable affinity for the NMDA receptor itself, it is likely
that the source of interaction between PCP and clozapine-like
antipsychotics is at a systems level through the various
neurotransmitter systems that are indirectly activated by PCP (Moore
et al., 1993; Coward, 1992; Saller and Salama, 1993). Clozapine has antagonistic properties at receptors within all of the
neurotransmitter systems that are indirectly activated by PCP, which
indicates that there are multiple possible sites of interaction between
the effects of these two compounds. Thus, identification of a
receptor-selective antagonist that blocks the PPI-disruptive effects of
PCP would indicate one source of interaction between the clozapine-like
antipsychotics and PCP. That is, whereas the primary mechanism of PCP
action is presumed to involve the NMDA receptor, clozapine-like
antipsychotics may act by blocking a secondary effect of PCP distal to
the relevant NMDA receptors.
Previous studies of selective antagonists for D1, D2, and 5-HT2
receptors have shown these compounds to be ineffective in blocking the
deficit in PPI produced by PCP (Bakshi et al., 1994), which
suggests that these receptors are probably not responsible for the
interaction between clozapine and PCP. In addition to its actions on DA
and 5-HT receptors, clozapine also has a high affinity for
alpha-1, alpha-2 and muscarinic M1 receptors
(Coward, 1992). Furthermore, studies of the GABA ion channel indicate
that clozapine functions as a GABA-A antagonist in vitro
(Squires and Saedrup, 1993). It has yet to be determined, however,
whether antagonists at these additional receptors will prevent
PCP-induced disruptions of PPI. The purpose of the present
investigation was to determine whether alpha-1,
alpha-2, M1 or GABA-A receptors might be involved in the
interaction between clozapine-like antipsychotics and PCP.
Specifically, we tested whether selective antagonists for
alpha-1, alpha-2, M1 or GABA-A receptors (given
individually) would block PCP-induced deficits in PPI.
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Materials and Methods |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Animals. A total of 338 male Sprague-Dawley rats (Harlan Laboratories, San Diego, CA), weighing 300 to 400 g, were used in the present studies. Animals were housed in pairs in clear plastic cages located inside a temperature- and humidity-controlled animal colony and were maintained on a reversed day/night cycle (lights on from 7:00 P.M. to 7:00 A.M.). Food (Harlan Teklad, Madison, WI) and water were available continuously except during behavioral testing, which occurred between 9:00 A.M. and 5:00 P.M. Upon arrival in the colony, all animals were handled gently by the experimenter in order to minimize stress during behavioral testing. Animal facilities were AAALAC-approved; protocols were in accordance with the "Guiding Principles in the Care and Use of Animals" (provided by the American Physiological Society) and the guidelines of the National Institutes of Health.
Drugs. The following drugs were used: PCP hydrochloride (1.5 mg/kg); apomorphine hydrochloride (0.5 mg/kg), both from Sigma Chemical Co. (St. Louis, MO); prazosin hydrochloride (0.5, 1.0 or 2.5 mg/kg); RX821002 hydrochloride (0.2 or 0.4 mg/kg); pirenzepine dihydrochloride (10 or 30 mg/kg), all from (Research Biochemicals International Natick, MA) and pitrazepin (1.0 or 3.0 mg/kg) (Novartis Pharmaceuticals, Basel, Switzerland). PCP, RX821002, pirenzepine and pitrazepin were dissolved in isotonic saline. Apomorphine was dissolved in saline with 0.1% ascorbic acid. Prazosin was dissolved in either a vehicle of distilled water and dimethyl sulfoxide (5%) or a vehicle solution of saline (50%), propylene glycol (40%) and ethanol (10%). All doses were calculated as the salt. Injection volume was 1 ml/kg for all drugs except pitrazepin, which was administered in a volume of 2 ml/kg.
Apparatus.
All testing occurred within startle chambers
acquired from San Diego Instruments (San Diego, CA). Startle boxes
consisted of clear nonrestrictive Plexiglas cylinders resting on a
platform inside a ventilated and illuminated chamber. A high-frequency loudspeaker (Radio Shack Supertweeter, San Diego, CA) inside the chamber produced both a continuous background noise of 65 dB and the
various acoustic stimuli. As described previously (Mansbach et
al., 1988), the whole-body startle response of the animal caused vibrations of the Plexiglas cylinder, which were then converted into
analog signals by a piezoelectric unit attached to the platform. These
signals were then digitized and stored by a microcomputer and interface
unit. Weekly calibrations were performed on the chambers to ensure the
accuracy of the sound levels and measurements. Sound levels were
measured as described previously (Mansbach et al., 1988)
using the dB(A) scale.
Behavioral testing.
One week after arrival, all rats
underwent a brief startle session in order to create matched treatment
groups. In this session and the subsequent test session, the background
noise (65 dB) was presented alone for 5 min and then continued
throughout the remainder of the session. A total of 20 trials were
presented in a pseudo-random order: 17 presentations of a 40-msec
120-dB broadband burst and 3 trials in which a 77-dB burst preceded the 120-dB burst by 100 msec. Treatment groups were established by using
the mean startle response to the 120-dB PULSE-ALONE trial so that all
groups had comparable base-line startle reactivity. One to two days
after the base-line session, drug testing began. The test session
utilized in all of the experiments contained five different trial types
and had a total duration of 20 min: a PULSE-ALONE trial in which a
40-msec, 120-dB broadband burst was presented; three PREPULSE + PULSE trials in which 20-msec noises that were 3, 6 or 12 dB above the
background noise were presented 100 msec before the onset of the 120-dB
pulse and a NO STIMULUS trial, which included only the background
noise. All trial types were presented several times in a pseudo-random
order for a total of 52 trials (20 PULSE-ALONE trials and 8 each of the
remaining trial types). In addition, 4 PULSE-ALONE trials, which were
not included in the calculation of PPI values, were presented at the
beginning of the test session to achieve a relatively stable level of
startle reactivity for the remainder of the session (based on the
observation that the most rapid habituation of the startle reflex
occurs within the first few presentations of the startling stimulus
(Geyer et al., 1990)). An average of 15 sec (ranging from 9 to 21 sec) separated consecutive trials.
Experimental design.
Six experiments were conducted using
separate groups of animals. In all experiments with PCP, the dose
utilized was 1.5 mg/kg (s.c., 10 min before entry into startle
chambers). This PCP regimen was chosen because it has been found
previously to produce a robust and reliable deficit in PPI that can be
antagonized by atypical antipsychotics (Bakshi and Geyer, 1995;
Swerdlow et al., 1996).
) would block PCP-induced deficits in PPI. Rats were pretreated
i.p. with either vehicle or 1.0 or 2.5 mg/kg prazosin and 20 min later
were given either saline or PCP. Because both prazosin doses were found
to antagonize the effects of 1.5 mg/kg PCP, we carried out a separate
experiment in order to evaluate a lower dose of prazosin. Thus, in
experiment 2, either vehicle or 0.5 mg/kg prazosin was administered
using the same protocol as in experiment 1.
Experiment 3 tested the hypothesis that prazosin antagonized PPI
disruptions in PCP-treated animals by producing a generalized improvement in deficient PPI. Thus the possibility that prazosin would
prevent deficits in PPI produced by a DA agonist (apomorphine) was
examined. Animals were pretreated i.p. with either vehicle or 1.0 or
2.5 mg/kg prazosin and 20 min later were treated with either vehicle or
apomorphine (0.5 mg/kg s.c.). This dose of apomorphine was selected
because it has been shown repeatedly to produce a disruption of PPI
that is antagonized by clozapine and other antipsychotics (Mansbach
et al., 1988; Swerdlow et al., 1991). Rats were
placed into startle chambers immediately after the second injection.
In order to assess the receptor subtype selectivity of the prazosin
effect, we examined in experiment 4 the possibility that an
alpha-2 antagonist, RX821002 (Langin et al.,
1989), would reverse the PCP-induced PPI deficit. Either saline or 0.2 or 0.4 mg/kg RX821002 was injected i.p. 20 min before PCP
administration.
Experiment 5 tested the possibility that a M1 muscarinic antagonist,
pirenzepine (Luthin and Wolfe, 1984), would prevent the PCP-induced
deficit in PPI. Rats were pretreated i.p. with either vehicle or 10 or
30 mg/kg pirenzepine and 20 min later were given either saline or PCP.
In the final experiment, we evaluated the effects of a GABA-A
antagonist, pitrazepin (Gahwiler et al., 1984), on
PCP-induced PPI deficits. The rationale for this study was that
clozapine, which antagonizes the PCP-induced PPI deficit (Bakshi
et al., 1994), has in vitro effects on the GABA
chloride channel that are identical to the effects of selective GABA-A
antagonists (Squires and Saedrup, 1993). Thus experiment 6 sought to
determine whether the antagonism by clozapine of the PCP effects on PPI
occurred via an indirect interaction through the GABA-A
system. Pitrazepin (vehicle or 1.0 or 3.0 mg/kg i.p.) was administered
20 min before either saline or PCP.
Data analysis.
The startle response to the 120-dB burst was
recorded for each PULSE-ALONE and PREPULSE + PULSE trial. Two
measures were calculated from these data for each animal. First, the
amount of PPI was calculated as a percentage score for each
PREPULSE + PULSE trial type: % PPI = 100 
{[(startle response for PREPULSE + PULSE trial)/(startle response for PULSE-ALONE trial)] × 100}. Second, startle magnitude was calculated as the average response to all of the PULSE-ALONE trials. All PPI data were analyzed with three-factor analysis of
variance (ANOVA) with pretreatment and treatment as between-subjects factors and trial type (prepulse intensity) as a repeated measure. Startle magnitude data were analyzed with two-factor (pretreatment and
treatment) ANOVA. Post-hoc analyses were carried out using Tukey's test. P level was set at .05.
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Results |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Effect of prazosin on PCP-induced behavior.
Experiment 1 yielded several important results that are illustrated in figure
1. First, administration of PCP resulted
in an abolition of PPI, evidenced by a statistically significant main
effect of treatment [F(1,36) = 50.88, P < .001].
Subsequent analyses indicated that PCP disrupted PPI at all three
prepulse intensities (P < .01). A main effect of pretreatment was
also observed [F(2,36) = 5.80, P < .007], which is
consistent with the small augmentation of PPI produced by prazosin at
all three prepulse intensities (fig. 1). Post-hoc analyses,
however, failed to reveal a statistically significant difference
between either dose of prazosin and the vehicle condition in
saline-treated animals at any prepulse intensity. Interestingly, a
nearly significant prepulse intensity × pretreatment × treatment interaction was obtained through ANOVA [F(4,72) = 2.36, P < .061]. As depicted in figure 1, prazosin pretreatment
increased PPI in PCP-treated animals by as much as 300%, nearly
returning PPI in these animals to control values. Given the magnitude
of this effect, the a priori hypothesis for this experiment,
and the significant main effects of treatment and pretreatment, we
conducted post-hoc analyses. The results of these subsequent
analyses confirmed the blockade of the PCP-induced deficit by prazosin.
Animals that received the lower dose of prazosin in conjunction with
PCP exhibited significantly higher levels of PPI at both the 6-dB
(P < .01) and the 12-dB (P < .05) prepulse intensities than
animals that received only PCP. Similarly, animals that were pretreated
with the 2.5 mg/kg dose before PCP administration also had greater PPI
than their vehicle-pretreated controls (P < .05, 12-dB prepulse
intensity). Thus prazosin was found significantly to antagonize the
deficit in PPI produced by PCP.
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Effect of prazosin on apomorphine-induced behavior.
Experiment
3 assessed the effects of prazosin on apomorphine-induced deficits in
PPI. The purpose of this experiment was to test the hypothesis that the
antagonism of the PCP effect by prazosin might simply be attributed to
a generalized improvement of deficient PPI. Analysis of variance
revealed a significant main effect of apomorphine treatment
[F(1,36) = 98.92, P < .001], which confirmed that
apomorphine disrupted PPI, as illustrated in figure
3. This effect was observed at the 3- (P < .01), 6- (P < .05) and 12- (P < .01) dB prepulse
intensities. Thus, as was observed with PCP, apomorphine produced a
nearly complete loss of PPI in vehicle-pretreated animals. In contrast,
no main effect of prazosin pretreatment was observed
[F(2,36) = 0.34, N.S.]. Furthermore, in contrast to the
two PCP studies, no significant interactions between pretreatment and
treatment were seen [F(2,36) = 0.55, N.S.], which
indicates that prazosin failed to antagonize the apomorphine-induced
deficit in PPI.
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), that was
produced by apomorphine (table 2).
Analysis of variance indicated a main effect of treatment
[F(1,36) = 25.10, P < .001]. Post-hoc analyses revealed that apomorphine-treated animals had higher NO
STIMULUS scores than vehicle-treated controls (P < .01). In contrast, no main effect of pretreatment was observed. Animals that
received either dose of prazosin before apomorphine had lower scores
than those that received only apomorphine (roughly a 50% reduction),
which indicates that prazosin also antagonized increases in overall
activity that were produced by apomorphine. It should be noted that no
effects on this measure were observed in any of the other studies and
that therefore those data are not presented. Finally, a separate
apomorphine study was conducted with a longer (30-min) pretest
injection interval for prazosin. This experiment yielded the same
pattern of results (prazosin blocked the increases in startle magnitude
and motor activity produced by apomorphine but had no effect on the
apomorphine-induced PPI deficit) as those described for experiment 3, so, for the sake of brevity, those data are not presented here.
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Effect of alpha-2 antagonist on PCP-induced
behavior.
Figure 4 illustrates the
effects of RX821002 on PCP-induced deficits in PPI. A main effect of
treatment was found with ANOVA [F(1,42) = 26.58, P < .001], which confirmed again that PCP decreased PPI. Subsequent
analyses indicated that animals that received PCP had significantly
lower levels of PPI at the 3-dB and 6-dB prepulse intensities (P < .05) than animals that received only saline. Neither a main effect
of pretreatment [F(2,42) = 0.62, N.S.] nor a
pretreatment × treatment interaction [F(2,42) = 1.19, N.S.] was observed. Thus, in contrast to the alpha-1
antagonist prazosin, the alpha-2 antagonist RX821002 had no
effect on PPI in PCP-treated animals.
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Effect of M1 muscarinic antagonist on PCP-induced behavior.
The effects of pirenzepine on PCP-induced deficits in PPI are depicted
in figure 5. As in the previous studies,
a significant main effect of treatment was revealed by ANOVA
[F(1,36) = 40.26, P < .001]. No main effect of
pretreatment was observed [F(2,36) = 0.59, N.S.], but a
prepulse intensity × pretreatment × treatment interaction
was found [F(4,72) = 2.74, P < .036]. The probable source of this interaction was the (statistically insignificant) tendency of pirenzepine to augment PPI in saline-treated animals but
further decrease PPI in PCP-treated rats (fig. 5). Thus the M1 muscarinic antagonist also did not block the deficit in PPI produced
by PCP.
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Effect of GABA-A antagonist on PCP-induced behavior.
Figure
6 shows the results of the final
experiment. In experiment 6, PCP was again found to disrupt PPI, as
indicated by a main effect of treatment [F(1,42) = 39.9, P < .001] and significant post-hoc comparisons
between PCP- and saline-treated groups (P < .01, all three
prepulse intensities). In contrast, neither a main effect of
pretreatment [F(2,42) = 1.60, N.S.] nor any interaction between factors was indicated by ANOVA. Thus PCP disrupted PPI, but
this effect was not prevented by the GABA-A antagonist pitrazepin.
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Discussion |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Several important results were obtained from the present studies.
First, in accordance with many previous reports, PCP was found
repeatedly to disrupt PPI (Bakshi and Geyer, 1995; Bakshi et
al., 1994; Mansbach and Geyer, 1989; Swerdlow et al.,
1996; Wiley, 1994; Varty and Higgins, 1994; Johansson et
al., 1994b). Second, the selective alpha-1
noradrenergic antagonist prazosin prevented the PCP-induced deficit in
PPI. This effect was produced by multiple prazosin doses, was observed
over multiple prepulse intensities and was replicated in a second
separate experiment. Finally, selective antagonists for either
alpha-2 (RX821002), muscarinic M1 (pirenzepine) or GABA-A
(pitrazepin) receptors had no effect on the disruption of PPI produced
by PCP. Taken together, these results indicate strongly that the
impairment of sensorimotor gating induced by PCP involves the
presumably indirect activation of alpha-1 adrenergic
receptors, but probably not alpha-2, M1 or GABA-A receptors.
To the best of our knowledge, this is the first report of the blockade
of PCP-induced deficits in PPI by an antagonist of noradrenergic
receptors. The present findings are in agreement with a recent study
that found that locomotor hyperactivity produced by dizocilpine
(another noncompetitive NMDA antagonist that also disrupts PPI) is
prevented by prazosin in a dose range identical to that of the present
studies (Mathe et al., 1996). Furthermore, depletion of NE
by the selective neurotoxin
N-(2-chloroethyl-N-ethyl-2-bromobenzylamine) (DSP4) has been reported
to prevent both PCP- and amphetamine-induced disruptions in sensory
gating as assessed by auditory evoked potentials, which suggests that
noradrenergic systems could indeed play a role in central gating
mechanisms (Adler et al., 1988; Miller et al.,
1992). Deficits in auditory gating produced by PCP have also been
reported to be blocked by the nonselective alpha adrenergic antagonist phentolamine (Stevens et al., 1991). The results
of the present studies provide further strong support for the
hypothesis that the behavioral effects of noncompetitive NMDA
antagonists are mediated partially, though not necessarily directly, by
alpha-1 adrenergic receptors.
The prazosin-induced antagonism of PCP effects was not accompanied by
significant increases in base-line levels of PPI, nor was it altered by
calculating PPI as a difference score (PULSE-ALONE value PREPULSE + PULSE value) rather than as a percentage (data not
shown). In addition, the disruption of PPI that was produced by the DA
agonist apomorphine was not antagonized by prazosin, even at
the highest dose. These findings, taken together, indicate that the
antagonism of PCP-induced deficits in PPI by prazosin cannot be
attributed to a generalized improvement in either basal or deficient
PPI, nor can it be explained simply by changes in startle reactivity.
Thus the prazosin antagonism of the PPI deficit appears to be a
selective effect on sensorimotor gating abnormalities produced by PCP.
It could be suggested that the failure of prazosin to prevent
apomorphine-induced deficits in PPI was due to the utilization of doses
that were ineffective against this dose of apomorphine. However, the
efficacy of prazosin in antagonizing both the increase in startle
reactivity (table 1) and that in general activity (e.g., NO
STIMULUS scores, table 2) produced by apomorphine in the same animals
argues against this possibility. Furthermore, the antagonism by
prazosin of the apomorphine-induced increase in startle magnitude is
consistent with a previous report that utilized the same dose range as
in the present studies (Davis et al., 1985). The effects on
NO STIMULUS scores extend the earlier finding that apomorphine
increases this measure of motor activity and that this effect can be
antagonized by haloperidol (Mansbach et al., 1988).
In contrast to the results with prazosin, selective antagonists for
alpha-2, M1 and GABA-A receptors failed to affect
PCP-induced deficits in PPI. Although these compounds were without
effect in the present studies, it should be noted that the dose ranges utilized are sufficient to achieve bioactivity in other behavioral assays (Siviy et al., 1994; Bymaster et al.,
1993). The present set of results further extends the previous finding
that selective D1, D2, and 5-HT2 antagonists do not block the
disruption in PPI produced by PCP (Bakshi et al., 1994;
Keith et al., 1991) and suggests that the effects of
clozapine-like antipsychotics in reducing PCP-induced deficits in PPI
are probably not derived from antagonism of these receptors. This
notion is corroborated by the finding that Seroquel, a novel putative
atypical antipsychotic with relatively low affinity for muscarinic,
dopaminergic and serotonergic receptors compared with adrenergic
receptors (Bymaster et al., 1996), produces a robust
blockade of the disruption in PPI produced by PCP (Swerdlow et
al., 1996). On the other hand, the locomotor-activating effects of
PCP and other noncompetitive NMDA antagonists can be
prevented by DA antagonists, which suggests that the mechanisms that
bring about the antagonism of sensorimotor gating deficits are distinct
from those that mediate general behavioral activation (Hoffman, 1992).
Antipsychotics such as clozapine, olanzapine and Seroquel have been
reported to antagonize PCP-induced deficits in PPI (Bakshi et
al., 1994; Bakshi and Geyer, 1995; Swerdlow et al.,
1996). A shared feature of these compounds is a high affinity for
alpha-1 adrenergic receptors (Moore et al., 1993;
Coward et al., 1989; Saller and Salama, 1993). In fact, a
recent direct comparison of the alpha-1 binding profiles of
these compounds revealed that clozapine and Seroquel have identical
affinity for the alpha-1 receptor (Ki = 7 nM) and that this value is approximately half that of olanzapine
(Ki = 19 nM) (Bymaster et al., 1996),
which indicates that clozapine and Seroquel are potentially 2-fold more potent as alpha-1 antagonists than olanzapine.
Interestingly, the relative doses of these antipsychotics that are
required to antagonize PCP-induced deficits in PPI roughly correspond
to this ratio; clozapine and Seroquel exhibit blockade at a dose (5 mg/kg) that is half the optimal olanzapine dose (Bakshi et
al., 1994; Bakshi and Geyer, 1995; Swerdlow et al.,
1996). Seroquel has roughly a 3- to 10-fold higher affinity for
alpha-1 than for 5-HT2 receptors (Saller and Salama, 1993;
Bymaster et al., 1996). This difference is in contrast to
clozapine and olanzapine, which, depending on the assay conditions,
show either greater or equal affinity for 5-HT2 compared with
alpha-1 sites (Moore et al., 1994; Leysen et al., 1993). Previous research has suggested that 5-HT2
antagonists may worsen the PPI deficit produced by PCP (Bakshi et
al., 1994). In conjunction with the present finding of
prazosin-induced blockade of PCP-induced deficits, this neurochemical
profile raises the possibility that the alpha-1 component of
certain antipsychotics is responsible for their antagonism of
PCP-induced deficits in PPI but that the 5-HT2 component, which is more
pronounced with clozapine and olanzapine, mitigates the
alpha-1 effect, resulting in a partial rather than full
blockade of the PCP effect. Indeed, Seroquel has been shown to produce
a nearly complete antagonism of the PCP-induced deficit in PPI,
mirroring the effect of prazosin in the present studies, whereas
clozapine and olanzapine antagonize the loss of PPI only by roughly
50% (Bakshi et al., 1994; Bakshi and Geyer, 1995; Swerdlow
et al., 1996). Future studies correlating central receptor
occupancy with efficacy for blocking PCP-induced deficits in PPI are
needed to test this hypothesis directly. Nonetheless, the present
findings strongly suggest that clozapine-like antipsychotics might
reduce PCP-induced deficits in PPI in large part through antagonism of
alpha-1 adrenoceptors. In contrast, clozapine, olanzapine and Seroquel seem to block apomorphine-induced deficits in PPI not
through alpha-1 antagonism, but rather via the
blockade of DA receptors, because apomorphine-induced deficits in PPI
are not blocked by prazosin but can be prevented by haloperidol
(Mansbach et al., 1988; Swerdlow et al., 1991).
One mystery that remains to be solved is why remoxipride, a relatively
selective D2 antagonist that does not possess appreciable
alpha-1 affinity, has been reported to block PCP-induced
deficits in PPI (Johansson et al., 1994a).
The present results provide solid evidence for an involvement of the
noradrenergic system in the PPI-disruptive effects of PCP. Future
studies using selective NE-depleting agents could examine the
obligatory nature of NE in this effect by investigating whether
PCP-induced deficits in PPI could be prevented by the loss of NE. In
addition, it will be of interest to determine whether direct
alpha-1 agonists disrupt PPI. The precise mechanism by which
PCP influences noradrenergic transmission and subsequently alters PPI
remains to be determined. PCP increases the firing of DA-containing
cells in the ventral tegmental area (French, 1994). Both the increase
in DA release and the enhancement of locomotor activity produced by
dizocilpine are antagonized by prazosin, which suggests that one mode
of functional interaction between noncompetitive NMDA antagonists and
noradrenergic systems occurs through the presynaptic modulation of
dopaminergic transmission (Svensson et al., 1995; Mathe
et al., 1996). Although the dose range of prazosin in the
present studies is similar to that used by Svensson and colleagues, it
is unlikely that the blockade of the PCP-induced deficit in PPI is
mediated by this DA-dependent mechanism, because, in contrast to
PCP-induced hyperactivity, PCP-induced disruptions of PPI cannot be
reduced by DA antagonism (Bakshi et al., 1994; Swerdlow
et al., 1996; Keith et al., 1991; French and
Vantini, 1984). Some of the anatomical sites that mediate the
disruption of PPI by noncompetitive NMDA antagonists have been
determined (Bakshi and Geyer, 1996), but the brain regions subserving
the blockade of this disruption by prazosin remain to be identified.
Studies are currently being conducted to delineate the specific
neuroanatomical circuitry involved in the interaction between
noncompetitive NMDA antagonists and alpha-1 receptors in the
modulation of PPI.
It is interesting to note that elevations in NE have been found in the
cerebrospinal fluid, plasma or brain tissue of schizophrenia patients,
an observation that offers some evidence for disturbances of the
noradrenergic system within this psychiatric population (Bird et
al., 1979; Breier et al., 1990; Farley et
al., 1978; Hornykiewicz, 1982; Kemali et al., 1990;
Kleinman et al., 1979; van Kammen et al., 1989).
One clinical study, which presumably tested the notion that a state of
hypernoradrenergia might be involved in schizophrenia, examined the
effects of alpha-1 receptor blockade by prazosin in
schizophrenia patients, but did not reveal a significant amelioration
of psychotic symptoms (Hommer et al., 1984). It should be
noted, however, that the sample was relatively small and included only
patients who were subsequently found to improve with administration of
"traditional" antipsychotics (e.g., DA antagonists, or
neuroleptics). Given that some schizophrenia patients do not respond to
DA antagonists but do respond to atypical antipsychotics such as
clozapine (Kane et al., 1988), it is possible that different
neural substrates are involved in neuroleptic-responsive than in
neuroleptic-refractory schizophrenia. Indeed, it may be that some
property other than DA receptor antagonism underlies the therapeutic
efficacy of clozapine in neuroleptic-resistant schizophrenia patients.
Actions at alpha adrenergic systems have been suggested
previously to be an important component in the effects of certain
antipsychotic medications (Baldessarini et al., 1992;
Breier, 1994; Cohen and Lipinski, 1986; Prinssen et al.,
1994; Svensson et al., 1995). The results of the present studies provide further evidence for a critical role of the
noradrenergic system, and in particular the alpha-1
receptor, in some of the behavioral effects of certain antipsychotics.
The present studies show that, like clozapine and related
antipsychotics, prazosin prevents PCP-induced PPI deficits. It is
possible that the alpha noradrenergic mechanism implicated
by the present findings is important in the gating abnormalities
exhibited by only a particular subset of schizophrenia patients.
Therefore, it could be predicted that prazosin might be effective in
treating this subpopulation of patients, even though it failed to
ameliorate the symptoms of neuroleptic-responsive patients (Hommer
et al., 1984). Thus future studies might explore its
potential utility in the treatment of either certain
neuroleptic-resistant schizophrenia patients or in PCP-induced
psychotic states.
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Acknowledgments |
|---|
The authors wish to express their gratitude to Elizabeth Lutz, Darlene Giracello and Beth Gregersen-Coates for their excellent technical assistance. In addition, we thank Dr. Richard Squires for the stimulating and helpful discussions about pitrazepin. M.A.G. holds an equity position in San Diego Instruments, Inc.
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Footnotes |
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Accepted for publication July 25, 1997.
Received for publication March 6, 1997.
1 This work was supported in part by Grant MH42228 from the National Institute of Mental Health and Grant DA02925 from the National Institute on Drug Abuse.
2 Supported by Grant F31-MH11636 from the National Institute of Mental Health.
3 Supported by a Research Scientist Award (K05-MH01223) from the National Institute of Mental Health.
Send reprint requests to: Mark A. Geyer, Department of Psychiatry, 0804, University of California at San Diego, La Jolla, CA 92093-0804.
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Abbreviations |
|---|
PCP, phencyclidine;
PPI, prepulse inhibition;
NMDA, N-methyl-D-aspartate;
DA, dopamine;
NE, norepinephrine;
5-HT, serotonin;
GABA, -aminobutyric acid;
M1, muscarinic 1;
AAALAC, Association for the Assessment and Accreditation of Laboratory Animal
Care.
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References |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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  A. E. Khalifa Neural monoaminergic mediation of the effect of St. John's wort extract on prepulse inhibition of the acoustic startle response in rats J Psychopharmacol, September 1, 2005; 19(5): 467 - 472. [Abstract] [PDF]
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 D. C. Goff and J. T. Coyle The Emerging Role of Glutamate in the Pathophysiology and Treatment of Schizophrenia Am J Psychiatry, September 1, 2001; 158(9): 1367 - 1377. [Abstract] [Full Text] [PDF]
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